What is the approximate chemical compositon with which all stars are born?

Three quarters hydrogen, one quarter helium, and no more than 2% heavier elements.

The total amount of power that a star radiates into space is called its _____.

Luminosity.

According to the invers square law of light, how will the apparent brightness of an object change if its distance to us triples?

Its apparent brightness will decrease by a factor of 9.

Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?

Calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity.

If star A is closer to us than star B, then Sta A's parallax angle is _____.

Larger than that of Star B

Ten parsecs is about _____.

32.6 light-years.

Star A has and apparent magnitude of 3 and star B has an apparent magnitude of 5. Which star is brighter in our sky?

Star A.

From hottest to Coolest, the order of the spectral types of stars is _____.

OBAFGKM

OUr Sun is a star of spectral type_____.

G.

Astronomers can measure a star's mass in only certain cases. Which one of the following cases might allow astronomers to measure a star's mass?

The star is a member of the binary star system.

What term is given to a pair of stars that we can determine are orbitiong each other only by measuring their periodic Doppler shifts?

Spectroscopic binary.

The axes on a Hertzsprung-Russell diagram represent _____.

Luminosity and surface temperature.

What can we infer, at least roughly, from a star's luminosity class?

Its size.

On an H-R diagram, stellar radii _____.

Increase diagonally from the lower left to the upper right.

On an H-R diagram, stellar masses _____.

Can be determined for main sequence stars but not for other types of stars.

How is the lifetime of a star related to its mass?

More massive stars live much shorter lives than less massive stars.

What spectral type and luminosity is a red supergiant?

Spectral type M2, luminosity class I.

What is the common trait of all main sequence stars?

They generate energy through hydrogen fusion in their core.

Suppose our Sun were suddenly replaced by a supergiant star. What would be true?

Earth would be inside the supergiant.

What is a white dwarf?

The remains of a star that ran out of fuel for nuclear fusion.

What statement comparing open and globular star clusters is not true?

Open and globular clusters each typically contain a few hundred stars.

What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?

It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age.

All stars are born with the same basic composition, yet stars can look quite different from one another. Which two factors primarily determine the characteristics of a star?

Its mass and its stage of life.

Based on the definition of apparent brightness, which units are appropriate for its measurements?

Watts per square meter.

Star A is identical to Star B, except that Star A is twice as far from us as Star B. Therefore:

Both stars have the same luminosity, but the apparent brightness of Star B is four times that of Star A.

A star with a parallax angle of 1/20 arcsecond is _____.

20 parsecs away.

The star Vega has an absolute magnitude of about 0. Based on the definitions of absolute and apparent magnitude, we can conclude that _____.

Vega is nearer than 10 parsecs from Earth.

What statement about spectral types of stars is not generally true?

The spectral type of a star can be used to determine its distance.

Sirius is a star with spectral type A star and Rigel is a star with spectral type B star. What can we conclude?

Rigel has a higher surface temperature than Sirius.

To calculate the masses of stars ina binary system, we must measure their_____.

Orbital period and average orbital distance.

Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes dimmer for about 2 hours. The most likely explanation is that _____.

The star is a member of an ecliosing binary star system.

You observe a star and you want to plot it on an H-R ndiagram. You will need not measure the star's _____.

Mass.

The approximate main-sequence lifetime of a star of spectral type O is _____.

3 million years.

How did astronomers discover the relationship between spectral type and mass for main sequence stars?

by measuring the masses and spectral types of main-sequence stars in binary systems.

What describes the youngest star cluster on the H-R diagram?

The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.

On the H-R diagram, which cluster is most likely to be located in the halo of our galaxy?

The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.